1. Field of the Invention
The present invention generally relates to an evaporation source to be used, for example, for an evaporation device for thin films, or the like, and particularly relates to an improvement in evaporating efficiency of the evaporation source.
The present invention also relates to a method of performing cluster-ion beam evaporation and it particularly relates to a method of forming a thin film by controlling the energy of cluster-ions.
2. Background of the Invention
FIG. 1 is a cross section showing a conventional evaporation source, for example, as disclosed in Japanese Patent Publication No. 54-9592. In the drawing is shown a crucible 101. Heating filaments 102 heat the crucible 101. A cap 103 covers the crucible 101 with a cylindrical nozzle 104 being provided in the cap 103. An evaporation material is contained in the crucible 101 and emits a vapor 106 of the evaporation material 105. The vapor 106 coalesces into clusters 107 of the evaporation material 105 to produce a beam 108 of the clusters 107, which impinge on a substrate 109.
Next, the operation will be described In the above-mentioned structure, the heating filaments 102 are electrically energized to generate heat so as to heat the crucible 101 by means of the radiation from the heating filaments 102 or by means of electron bombardment. That is, thermions emitted from the filaments 102 collide against the crucible 101, so that the evaporation material 105 in the crucible 101 is evaporated. When the pressure of the vapor 106 of the evaporation material 105 generated in the crucible 101 reaches 0.1-10 torr, the vapor 106 is jetted through the cylindrical nozzle 104. At that time, the vapor 106 is condensed by adiabatic expansion caused by a pressure differential, so that groups of massive atoms, each called a cluster, in which about several tens to several thousands of atoms in the vapor 106 are loosely coupled with each other to form a cluster 107 The clusters 107 are used for forming a thin film on the substrate 109.
FIG. 2 shows a result of measurement of a film thickness distribution of a thin film of Ag formed on a substrate by using such a conventional evaporation source as described above.
The abscissa in the drawing represents the position on the substrate in term of tan8 where 8 represents a jetting angle of vapor. The result is shown by experimental values in the case where the distance between the substrate and the nozzle was set to 100 mm.
As will be apparent from FIG. 2, the film thickness distribution of a thin film formed by evaporation by using the conventional evaporation source is sharp, and there has been such a problem that a uniform range of the film thickness is narrow For example, the range having a film thickness of 80% or more of the maximum film thickness at the evaporation center extends only to tan.theta.=0.25, that is, up to only about 14 degrees of the jetting angle .theta.. An experimental value of the evaporation efficiency in the case of using this range for evaporation was about 17%, and therefore there has been such a problem that the evaporation efficiency is poor when the conventional evaporation source is used.
Generally, a method of forming a thin film through evaporation by using a cluster-ion beam evaporation method is performed through a process in which, in a vacuum chamber, a vapor of the material to be deposited through evaporation onto a substrate is jetted to generate clusters (groups of massive atoms) each composed of a number of loosely coupled atoms in the vapor. Electrons are showered upon the clusters so as to ionize one of the atoms of the respective cluster to make the cluster to be a cluster-ion. The cluster-ions are accelerated to collide against the substrate to thereby form a thin film on the substrate through evaporation. As a device for carrying out thin film formation through the evaporation method, there is known a device, for example, as disclosed in Japanese Patent Publication No. 54-9592 and as shown in FIGS. 3 and 4. FIG. 3 is a cross-section diagrammatically showing a conventional thin film evaporation device and FIG. 4 is a perspective view, partly in section, showing a main portion of the device. In the drawings, a vacuum chamber 1 is kept at a predetermined degree of vacuum, and an exhaust path 2 is connected to a not-shown vacuum exhausting device for exhausting the vacuum chamber 1. A vacuum valve 3 opens and closes the exhaust path 2. A closed crucible 4 is provided with a nozzle 26 having a diameter of 1 mm to 2 mm and accommodates therein an evaporation material 5, for example, silver (Ag), a heating filament 6 heats the crucible 4. A heat shielding plate 7 blocks radiant heat from the filament 6. An evaporation source 8 is composed of the crucible 4, the heating filament 6, and the heat shielding plate 7. The evaporation source 8 jets the evaporation material 5 into the vacuum chamber 1 to generate clusters. An insulating support member 19 supports the heat shielding plate 7. A support stage 20 supports the crucible 4. An insulating support member 25 fixes the support stage 20 to the vacuum tank.
Ionizing filaments 9 emit thermions 13b for ionization. Grid electrodes 10 accelerate the thermions 13b emitted from the ionizing filaments 9. A heat shielding plate 11 blocks radiant heat from the ionizing filaments 9. Ionizing means 12 constituted by the ionizing filaments 9, the grid electrodes 10, and the heat shield plate 11, ionizes the clusters generated from the evaporation source 8. An insulating support member 23 supports the heat shielding plate 11. An accelerating means 14, that is, an accelerating electrode, accelerates the ionized cluster-ions 16 to cause the cluster-ions 16 together with the non-ionized neutral clusters 15 to collide against a substrate 18 on which a thin film is to be formed to thereby deposit a thin film through evaporation. The accelerating means 14 is arranged so that a potential can be applied between the accelerating means 14 and the grid electrodes 10. An insulating support member 24 supports the accelerating electrode 14. A substrate holder 22 supports the substrate 18. An insulating support member 21 supports the substrate 22. A cluster beam 17 composed of the cluster-ions 16 and the neutral clusters 15 impinges on the so-supported substrate 18.
Next, the thin film forming method employing the device as described above will be described.
The case will be described in which a silver thin film is formed through evaporation. First, the crucible 4 is filled with the silver 5 and the air in the vacuum tank 1 is pumped out by the vacuum exhausting device to keep the inside of the vacuum tank 1 at a vacuum of about 10.sup.-6 torr. Next, the heating filaments 6 are energized to generate heat, so that the silver 5 in the crucible 4 is heated to evaporate owing to the radiant heat from the heating filaments 6 or owing to collision of the thermions 13a emitted from the filaments 6 against the crucible 4, that is, electron-bombardment. When the temperature in the crucible 4 rises to a value at which the vapor pressure of the silver 5 reaches about 0.1 to several tens of torr, the vapor jetted from the nozzle 26 is adiabatically expanded because of a difference in pressure between the crucible 4 and the vacuum tank 1 to form groups of massive atoms each called a cluster in which number of atoms are loosely coupled with each other.
Because the cluster-beam 17 collides with the thermions 13b drawn out of the ionizing filaments 9 by the grid electrodes 10, one of the atoms in each of the clusters is ionized to make the clusters be the cluster-ions 16. The cluster-ions 16 are suitably accelerated in the electric field formed between the accelerating electrode 14 and the grid electrodes 10 so as to cause the cluster-ions 16 to collide against the substrate 18 together with the neutral clusters 15 which are simultaneously caused to collide against the same substrate 18 by means of kinetic energy when the neutral clusters 15 are jetted from the crucible 4. As a result, a thin film of silver is deposited on the substrate 18 through evaporation.
Thus, the conventional cluster-ion beam evaporation method is featured in that the cluster-ions are of monovalent ions, and composed of numbers of atoms numbering about 10.sup.2 to 10.sup.3. Furthermore, the charge-to-mass ratio of the cluster-ions is extremely small. Accordingly, a large quantity of evaporation material can be transferred with such a relatively low current density that the material is hardly affected by space charges, so that a thin film can be formed at a high evaporation speed. In order to use the advantage more effectively, it is necessary that the number of the atoms constituting each cluster, that is, the cluster size, is increased as much as possible.
In the conventional thin film forming method, however, the cluster size hardly depends on the size of the cylindrical nozzle but depends on the vapor pressure of the evaporation material. That is, a large cluster size is obtained only under the condition that the vapor pressure is within an extremely limited range. Accordingly, if it is intended to increase the quantity of the evaporation material jetted from the nozzle to thereby make the evaporation speed higher under the condition that the vapor pressure is made higher than the optimum value, the cluster size becomes small and the total number of the clusters are increased, so that the cluster-ions also increase correspondingly. Therefore, sometimes, the cluster beam diverges owing to the influence of the space charge so that a sufficient amount of cluster-ions cannot reach the substrate.
Further, sometimes, in the case where the vapor pressure of the evaporation material is changed to change the evaporation speed even under the condition that the material is hardly affected by the space charge, the energy of the cluster-ions is changed so that the characteristics of the thin film formed on the substrate is changed That is, in the conventional thin film forming method, there is such a disadvantage that the energy of the cluster-ions cannot be controlled to a desired value because the cluster size cannot be changed in accordance with the vapor pressure of the evaporation material.
In the device for forming the thin film through the conventional method, as described above, the thermions used for heating the crucible also irradiate the cluster-beam, so that the cluster-ions can be formed in the evaporation source and the quantity of the cluster-ions cannot be independently controlled by the ionizing means Because the ions formed in the evaporation source tend to concentrate at a center of the substrate, there has been such a disadvantage that the ion current density on the substrate is made uneven to thereby deteriorate the evenness of the thin film deposited through evaporation.
On the other hand, if electrons are prevented from bombarding against the cap portion of the crucible to prevent the cluster-ions from being formed in the evaporation source, the cap portion, particularly in the vicinity of the nozzle, is not sufficiently heated, so that the temperature is lowered. Therefore, the vapor condenses in the vicinity of the nozzle to generate liquid drops of the evaporation material, so that the quantity of the vapor to be used for forming the clusters is reduced or the liquid drops are scattered to damage the thin film formed on the substrate.
In the conventional method, when the film thickness distribution of the thin film formed on the substrate is to be changed, the diameter of the nozzle is changed without changing the thickness of the cap, that is, the length of the nozzle, so as to maintain constant the heating state of the crucible cap constant Accordingly, as the diameter of the nozzle changes, the evaporation speed is also changed In other words, the film thickness distribution cannot be changed with the evaporation speed kept constant.
Japanese Patent Publication No. 54-9592 discloses a method for vapor deposition, in which evaporation material are ejected through a nozzle into a high vacuum to form clusters. The cluster is ionized and accelerated. There is no detailed description in the publication with respect to the shape of the nozzle.
Further, there is a prior publication "Ionized-Cluster Deposited on a Substrate and Method of Depositing Ionized Cluster on a Substrate" described in U.S. Pat. No. 4,152,478. In FIGS. 17 and 18 of the U.S. Patent, the nozzle outlet is delineated as being tapered. However, there is no specific disclosure with respect to the shape and function of the nozzle.
Furthermore, there is a prior publication "Molecular Beams and Low Density Gasdynamics" edited by Peter P. Wegener. In this publication, a conical nozzle is used to form gaseous clusters, and description is found with respect to the relationship between the cluster size and shape and dimension of the nozzle.
Furthermore, in Japanese Patent Publication No. 56-25772, a thin film is formed on a silicon substrate by means of cluster ion beam method In the drawings, a taperednnozzle is shown. However, no specific description is found with respect to the shape of the nozzle.